The developmental mutants of Drosophila melanogaster have proven to be a rich and productive area of study for investigations into basic mechanisms of development. Mutants have been isolated that transform one segment into another - for example whole legs replace antennae in the heads of Antennapedia flies - suggesting that it is the function of such genes to decide whether to adopt a particular developmental pathway. Other mutants have been isolated that reduce the number of segments, that alter the polarity of the segments, or in the case of the maternal-effect bicaudal alleles, that alter the polarity of the egg and create an embryo with two posterior ends. Such phenotypes suggest processes that organize and subdivide the young embryo. How the products of these many different genes cooperate to fashion the normal embryonic patterns is not known. However, it is commonly believed that the maternal-effect genes function during oogenesis and during the early, pre-cellular stages of embryogenesis to organize the embryo. It is believed that these genes operate to define embryonic polarity and positional coordinates and to provide sufficient information for cellular blastoderm formation. Since very little gene expression has been detected prior to cellularization, it has been assumed that the role of the zygotically active genes in determining segment number and type is important only after cellularization. In the work proposed here, we will challenge this assumption: we will directly determine the stage at which zygotically active genes have a controlling role in development. Based upon our recent discovery that a number of mutations in zygotically-acting genes affect morphogenesis during the pre-cellular stages, we already know that our previous conceptions about the relative contributions of the maternally- and zygotically-acting genes must be re-evaluated.